Low parasitic capture fuel assembly structure

ABSTRACT

The basic fuel assembly structure for a nuclear reactor comprises a skeleton made up of individual hollow tubes composed of a zirconium base alloy, such as Zircaloy, and held in place by Inconel or stainless steel grids disposed between stainless steel end nozzles at the top and the bottom of the structure. The tubes serve as guide thimbles for cylindrical control elements strategically located in a square array of fuel rods held in place by spring fingers on the &#39;&#39;&#39;&#39;egg-crate&#39;&#39;&#39;&#39; grids. The guide thimbles are attached to the top and bottom nozzles and the grids by mechanical joints without welding dissimilar metals.

United States Patent 1191 Patterson et al.

[ LOW PARASITIC CAPTURE FUEL ASSEMBLY STRUCTURE [75] Inventors: John F.Patterson, Murrysville;

Elmer A. Bassler, ,lra; Edward J. Choby, both of Irwin, all of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Mar. 21, 1972 [21] Appl. No.1 236,780

Related U.S. Application Data [63] Continuation of Ser, No. 825,889, May19, 1969.

[ 1 Feb. 12, 1974 Primary Examiner-Carl D. Quarforth AssistantExaminer-Roger S. Gaither Attorney, Agent, or Firm-A. T. Stratton [5 7]ABSTRACT The basic fuel assembly structure for a nuclear reactorcomprises a skeleton made up of individual hollow tubes composed of azirconium base alloy, such as Zircaloy, and held in place by Inconel orstainless steel grids disposed between stainless steel end nozzles atthe top and the bottom of the structure. The tubes serve as guidethimbles for cylindrical control elements strategically located in asquare array of fuel rods held in place by spring fingers on theegg-crate" grids. The guide thimbles are attached to the top and bottomnozzles and the grids by mechanical joints without welding dissimilarmetals.

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LOW PARASITIC CAPTURE FUEL ASSEMBLY STRUCTURE This is a continuation, ofapplication Ser. No. 825,889 filed May 19, I969.

BACKGROUND OF THE INVENTION This invention relates, generally, to fuelassemblies for nuclear reactors and, more particularly, to a fuelassembly having a relatively small amount of parasitic structuralmaterial therein.

In the past, welding has been utilized to attach stain less steel hollowtubes to the grids and end nozzles of a canless fuel assembly comprisinga square array of fuel rods and the strategically located hollow tubeswhich serve as guide thimbles for cylindrical control elements. The fuelrods areheld in place by spring fingers on the grids spaced between theend nozzles. With the use of a zirconium base alloy, such as Zircaloy,as a guide thimble in order to still further reduce the quantity of highneutron capture material in a fuel assembly, normal welding or brazingtechniques cannot be utilized because a brittle alloy is formed at theinterface between the dissimilar metals.

In order to make the basic fuel assembly structure,

which comprises a skeleton made up of individual Zircaloy guide thimblesheld by lnconel or stainless steel grids within stainless steel nozzlesat top and bottom, a means must be provided to firmly attach the guidethimbles to the support grids and the end nozzles without welding orbrazing two dissimilar metals. Furthermore, since Zircaloy becomessignificantly brittle as a result of irradiation and hydrogen pickupduring opera tion of a reactor, it is desirable to strengthen thecritical joints between the guide thimbles and end nozzles with a moreductile material, such as stainless steel.

An object of the invention is to provide a mechanical joint between theguide thimbles and the support grids of a reactor fuel assemblystructure which results in minimum pertubation to coolant flowingthrough the fuel assembly; allows the use of a thimble tube having thelargest possible diameter; does not have loose contacts or connections;is not subject to severe corrosive attack, and has adequate structuralstrength.

Another object of the invention is to provide a joint between thethimble tubes and end nozzles which, in addition to the foregoingcharacteristics, does not severely restrict coolant flow, and hasadequate ductility to resist impact forces during fuel loading andunloading operations and abnormal reactor operating conditions.

Other objects of the invention will be explained herein-after or will beapparent to those skilled in the art.

SUMMARY OF THE INVENTION In accordance with one embodiment of theinvention, the quantity of high neutron capture material in a canlessfuel assembly for a nuclear reactor is reduced by utilizing hollow tubescomposed of a relatively low neutron capture material, such as azirconium base alloy, to serve as guide thimbles for cylindrical controlelements strategically located in a square array of fuel rods held inplace by spring fingers on Inconel or stainless steel egg-crate gridsdisposed between stainless steel end nozzles at the top and the bottomof the fuel assembly structure. The guide thimbles are firmly attachedto the end nozzles and the support grids by mechanical joints withoutwelding or brazing two dissimilar metals. The tubes or thimblesareretained in stainless steel sleeves by integrated protrusions on thetubes, and the sleeves are brazed to Inconel or stainless steel grids toprovide the main structural skeleton of the basic fuel assembly. Theskeleton is attachedto the stainless steel end nozzles through stainlesssteel sleeves in a manner to permit longitudinal expansion,

thereby reducing the loading on the fuel assembly caused by differentialthermal expansion between components. Also, the joints between the guidethimbles and the end nozzles are strengthened since the sleeves do notbecome as brittle as a result of irradiation during operation of thereactor.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thenature and objects of the invention, reference may be had to thefollowing detailed description taken in conjunction with theaccompanying drawings, in which:

FIG; l is a view; in elevation, ofa nuclear reactor fuel assemblyembodying principal features of the invention, portions being brokenaway for clearness;

FIG. 2 is a view, in section, taken along the line IIII in FIG. ll;

FIGS. 3 and 4 are enlarged detail. views, in section of portions of thestructure shown in FIG. 1;

FIG. 5 is a view in section, taken along the line V-V in FIG. 4;

FIG. 6 is an isometric view of a portion of one of the grid members andone of the tubular members utilized in the fuel assembly;

FIG. 7 is an isometric view, similar to FIG. 6, showing a modified gridand tube assembly;

FIG. 3 is an enlarged detail view of a portion of the assembly shown inFIG. 7;

FIG. 9 is a view, in section, taken along the line IX-IX in FIG. 8;

FIG. 10 is an enlarged detail view, in section, showing one of thetubular members assembled in the top nozzle and the top grid of the fuelassembly;

FIG. 11 is an isometric view of one of the retaining sleeves utilized inthe fuel assembly;

FIG. I2 is a view, in section, of a modified tube and top nozzleassembly; and

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings,particularly'to FIGS. l and 2, a fuel assembly It) shown thereincomprises an upper end structure or nozzle I2, a lower end structure ornozzle 14, a generally square array of substantially parallel elongatedfuel elements or rods 16, only a few of which are shown to avoidconfusion, a plurality of elongated hollow tubular members 18%containing control elements 26 strategically located among the array offuel elements 116, a top grid member 22, a bottom grid member 24, and aplurality of intermediate grid members 26 longitudinally spaced alongthe tubular members 18. An instrumentation tube 27 is located at thecenter of the fuel assembly-Thefuel assembly 10 is of the canless type,such as described in a copending continuation application, Ser. No.304,292, filed Nov. 6, 1972 by R. J. Creagen and E. Frisch and assignedto the Westinghouse Electric Corporation.

As shown more clearly in FIG. 2, the upper end noz zle 12 is generallysquare in shape. The nozzle 12 comprises an end plate 13, an enclosure15 secured to the plate 13 and a flange 17 secured to the top of theenclosure 15. Leaf springs 19 are clamped to the flange 17 by cornerblocks 21 and bolts 23. The springs cooperate with a core plate (notshown) to permit expansion of the fuel assembly, but prevent theassembly from being moved upwardly by hydraulic forces. Dowel pins (notshown) extend into diagonally disposed openings 25 in the flange 17 toprovide lateral support for the fuel assembly. The plate 13 has aplurality of openings 28 therein to permit the circulation of thereactor coolant through the end nozzle. Likewise, the lower end nozzle14 is generally square in shape and has a plurality of openings thereinto permit the circulation of the reactor coolant. The upper ends of thetubular members 18 are disposed in openings in the upper end nozzle 12as will be described more fully hereinafter.

As shown more clearly in FIG. 6, each one of the grid structurescomprises a plurality of straps 30 which are interfitted to provide astructural network similar to an egg-crate and forming a plurality ofgenerally square openings or cells 32 through which the fuel rods 16 andthe tubular members 18 pass. As shown in FIGS. 3 and 4, the straps 30are provided with resilient fingers 34 which engage the fuel rods 16 andthe tubular members 18 to support them against lateral displacement.

As explained hereinbefore, the hollow tubes 18 serve as guide thimblesfor the cylindrical control elements 20. In order to reduce the quantityof high neutron capture material in the fuel assembly, the tubes 18 arecomposed of a relatively low neutron capture material, such as azirconium base alloy known as Zircaloy. The basic fuel assemblystructure consists of a skeleton made up of individual Zircaloy guidethimbles held by lnconel or stainless steel grids with stainless steelnozzles at the top and the bottom of the structure. In order to makesuch a structure, a means must be provided to firmly attach the Zircaloyguide thimbles to the Inconel or stainless steel support grids and thestainless steel end nozzles. In the past, welding has been used toattach stainless steel guide thimbles to the grids and end nozzles. Withthe use of Zircaloy as a guide thimble material, normal metallurgicalbonding techniques, such as welding or brazing, are not suitable becausea brittle alloy is formed at the interface between the dissimilarmetals. Furthermore, since Zircaloy becomes significantly brittle as aresult of irradiation and hydrogen pick up during operation of areactor, it is desirable to strengthen the critical joint between theguide thimbles and end nozzles with a more ductile material, such asstainless steel.

In accordance with the present invention, individual fuel rods areloaded into the spring clip grid structures and the end nozzles areattached. The individual Zircaloy guide thimbles are attached to the topand bottom end nozzles and the support grids by mechanical joints,thereby avoiding the welding or brazing of two dissimilar metals in thefuel assembly structure.

As shown in FIGS. 1 and 3, the lower end nozzle 14 is attached to thebottom grid member 24 and to the tubular members 18 by means ofstainless steel sleeves 36 and machine screws 38. A sleeve 36 surroundseach tubular member 18 at the opening 32 in the grid structure throughwhich the tube 18 extends. A metal plug 40 is pressed and welded intothe lower end of each tube 18.

The sleeve 36 has an internal shoulder 42 which engages the lower end ofthe plug 40. The sleeve 36 is brazed to the grid 24 which is composed oflnconel or stainless steel. When the screw 38 is threaded into the plugwith the head of the screw engaging the end nozzle 14, the lower end ofthe sleeve 36 is retained between plug 40 in the lower end of the tube18 and the end nozzle 14. Tight clearances between the head of the screw38, the hole in the lower nozzle 14, and the hole in the bottom of thegrid sleeve 36 locate the bottom of the thimble accurately. Properselection ofjoint dimensions and initial screw tension insures a tightjoint over a wide temperature range without danger of the materialyielding as a result of differential thermal expansion between Zircaloyand stainless steel.

The joint structure makes it possible to connect the main structuralskeleton to the lower end nozzle through the stainless steel sleevewhich is brazed to the bottom grid 24. Such a connection could besubjected to high impact loading during fuel assembly handlingprocedures. Stainless steel is less susceptible to becoming brittleunder reactor operating conditions than Zircaloy. Therefore, the fuelassembly is less likely to be damaged during dynamic loading or handlingthan is the case with other connections.

As shown in FIGS. 4, 5 and 6, each intermediate grid support 26 isretained in position by means of two stainless steel sleeves 44a and 44bsurrounding each one of the tubular members 18 in the opening 32 in thegrid structure. The sleeves 44a and 44b are brazed to the grid straps30. The tubular member 18 is expanded to form protrusions 46 integralwith the tube immediately above and below the grid sleeve. Thus, theprotrusions 46 engage the stainless steel sleeves which, in turn, arebrazed to the grid, thereby retaining the grid in position. As shown inFIG. 5, the stainless steel sleeves have an inside diameterapproximately equal to the distance between the individual grid strapsmaking up the grid. Four slots in each sleeve at intervals make itpossible to insert the sleeve into an individual grid cell as the gridis assembled for brazing. Two sleeves, 44a and 44b are provided for eachgrid cell. The sleeve structure permits the use of maximum diameterguide thimbles.

The tool which is utilized to expand the tube 18 is so constructed thatflats are obtained on 90 angles of the expanded protrusion as shown inFIG. 5. The flats reduce coolant flow pertubations near the fuel rodswhose center line is directly opposite the flats. The maximum extensionof the protrusions at the four corners of the grid cells block coolantflow at this point. This blockage has a favorable affect upon flowdistribution within the fuel assemblies.

An alternate method for attaching the grid members to the tubularmembers is shown in FIGS. 7, 8 and 9. A tab extension 48 on a grid strap30 is captured by a spot weld between the Zircaloy tube 18 and aZircaloy clip 50. The clip is stamped to provide a base 52 which fitsthrough a hole 54 punched in the grid strap tab 48. The fit between thebase and the hole is such that the clip is securely clamped to thethimble tube by the tapered sides on the base when the base is spotwelded to the thimble tube. The clip 50 is also spot welded to the tube18 above the grid tab to secure the clip in the event the lower weld issheared. In this structure the spot welding is performed on two memberswhich are composed of the same material, namely Zircaloy. As

shown, tabs 48 and clips 50 are provided on grid straps at oppositesides of the tube 18.

The upper end nozzle 12 is attached to the upper ends of the tubularmembers 18 in the manner shown in FIG. 10. The upper end of each tube 18is disposed in an opening 56 in the end plate 13 of the nozzle 12. Astainless steel sleeve 58 is brazed to the top grid 22. The insidediameter of the sleeve is approximately equal to the distance betweenthe individual straps making up the grid assembly. Four slots 60 locatedin the sleeve 58 at 90 intervals, as shown in FIG. 11, make it possibleto insert the sleeve into a grid cell as the grid is assembled forbrazing. During this assembly it is necessary to slightly deform a 90segment of the lower circular section of the sleeve in order to fit itbe tween the grid straps. The sleeve is subsequently restraightenedbefore brazing. The sleeve structure makes it possible to use a Zirealoythimble tube of a maximum diameter. The top grid 22 is mechanicallyattached to the thimble tube 18 by first expanding the tube at a pointjust below the lower end of the sleeve 58 to form a protrusion 62integral with the tube. Finally, boththe thimble tube and the sleeve 58are expanded outwardly at one or more places above the grid to formprotrusions 64.integral with the tube and the sleeve. The plate 13 iswelded at 66 to the upper end of the sleeve 58.

In this manner the grid 22, the tube 18 and the end nozzle 12 aremechanically connected by means of the stainless steel sleeve 58.Stainless steel sleeves are used to structurally connect the main fuelbundle to the upper nozzle because this material retains more ductilityunder reactor operating conditions than Zirealoy. As previouslyexplained, a Zirealoy connection could be subject to damage duringdynamic loading due to becoming brittle due to irradiation and hydrogenpick up during reactor operation. It will also be noted that themechanical connection permits longitudinal expansion of the tubes 18 bycompressing or tightening the protrusions in the tubes as the reactorreaches operat ing temperature. If proper dimensions are selected thetubes will be tight during operation of the reactor.

An alternate method for attaching the upper end nozzle 12 is shown inFIG. 12. An adapter sleeve 68 surrounds each tube 18 in the opening 56in the plate 13. The lower end of the sleeve 68 has an external flange70 thereon. A retainer sleeve 72 surround the adapter sleeve. Theretainer sleeve 72 has an internal flange 74 engaging the externalflange 70 on the adapter sleeve 68. A capture sleeve 76 extends betweenthe adapter sleeve and the retainer sleeve to engage the internal flange74 on the retainer sleeve. The adapter sleeve and the capture sleeve arecomposed of Zirealoy, or a metal similar to the metal of the tubularmember 18. The retainer sleeve 72. is composed of stainless steel or ametal similar to the metal of the end plate 13. The adapter sleeve 68 issecured to the tubular member 18 by metallurgical bonding or welding,and the capture sleeve 76 is secured to the adapter sleeve bymetallurgical bonding or welding. Likewise, the retainer sleeve 72 issecured to the end plate 13 by metallurgical bonding or welding. In allcases the welding is performed between similar metals.

Another alternate method of attaching the upper end nozzle 12 is showninFIG. 13. In this arrangement, the upper end nozzle is attached to thetop grid member 22 rather than to the guide thimbles 18. In thestructure illustrated, an extension 78 is provided on the top gridmember 22. The extension 78 may be formed integrally with the outerstraps of the grid structure or it may be a separate member welded tothe grid structure. The extension 78 is welded to the end plate 13 or tothe top enclosure 15 for the end nozzle which, in turn, is welded to theend plate, thereby attaching the top grid member to the end nozzle inspaced relation. Also, integral tabs or extensions 82 on the grid member22 engage protrusions 84 on the tube 18 both above and below the gridmember. In this manner axial loads imposed on the assembly aretransferred between the end nozzle and the guide thimbles through thegrid assembly and the swaged joints between the grid and the thimbles.The top end of each tube 18 is mated with a close fitting hole 56 in theend plate 13 which maintains the lateral alignment of the tube, butpermits free axial movement of the tube in the event of differentialthermal expansion between the tube and the grid and the end nozzle.

As explained hereinbefore, a suitable tool may be provided for expandingthe Zirealoy tubes to form the protrusions thereon which cooperate withthe grid and sleeve members surrounding the tubes. The tool may be soconstructed that all tubes or thimbles in a fuel assembly are expandedat a specific vertical level simultaneously, thereby obtaining astraight skeleton assembly.

From the foregoing description it is apparent that the inventionprovides a fuel assembly for a nuclear reactor in which the quantity ofhigh neutron capture material is reduced, the loading on the fuelassembly structure which is caused by differential thermal expansionbetween components is reduced, and the power capability of the fuelassembly is improved. The fuel assembly is so constructed that weldingor brazing, or other forms of metallurgical bonding, is not required atjoints between dissimilar metals.

Since numerous changes may be made in the abovedescribed structure anddifferent embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all subjectmatter contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

We claim:

l. A fuel assembly for a nuclear reactor comprising an array ofgenerally parallel elongated fuel elements, a plurality of elongatedhollow tubular members containing control elements strategically locatedamong the array of fuel elements, said tubular members being composed ofa relatively low neutron capture metal,

means separately attaching the opposite ends of said tubular members totop and bottom end nozzles respectively located at opposite endsthereof;

at least one grid located between said end nozzles and formed bycoacting straps which define multi- 'ple spaced openings through whichthe fuelelements and tubular members extend;

a device immovably positioned on each of said tubular members, saiddevice being located between said tubular means and the walls of saidstraps; and

means securing said device to the inner walls of said straps whichdefine the opening through which each tubular member extends, to therebyhold each tubular member in position in the fuel assembly.

2. The fuel assembly defined in claim 1, wherein the device comprises asleeve surrounding each tubular member.

3. The fuel assembly defined in claim 2, wherein each sleeve is composedof a metal similar to the metal of the grid straps, and the sleeve issecured to the grid straps by metallurgical bonding.

4. The fuel assembly defined in claim 3, wherein the grid straps aredisposed at right angles to form generally square openings, and eachsleeve has slots therein of a size sufficient to receive the strapsduring grid assembly to thereby hold the device and tubular memberimmovably in the grid straps.

5. The fuel assembly defined in claim 2, wherein spring means is securedto the inside surface of and along the length of said straps formingeach opening, said spring means being positioned to bear against eachtubular member and fuel element in the appropriate openings to preventtheir lateral displacement.

6. The fuel assembly defined in claim 1, wherein each tubular member isdeformed adjacent at least one end of said device to immovably fix thedevice on its tubular member.

7. The fuel assembly defined in claim 1, wherein the device is composedof a metal similar to the metal of the tubular members and is secured tothe tubular members by metallurgical bonding.

8. The fuel assembly defined in claim 7, wherein the grid straps haverecesses therein, and the device extends into said recesses.

9. The fuel assembly defined in claim 1, wherein said device comprisesan adapter sleeve surrounding each tubular member extending through thetop nozzle, said adapter sleeve having an external flange thereon, aretainer surrounding the adapter sleeve, said retainer having aninternal flange engaging the external flange on the adapter sleeve, anda capture sleeve extending between the adapter sleeve and the retainerto engage the internal flange on the retainer.

10. The fuel assembly defined in claim 9, wherein the adapter sleeve andthe capture sleeve are composed of a metal similar to the metal of thetubular member and the retainer is composed of a metal similar to theend nozzle; and wherein said adapter sleeve is secured to the tubularmember by metallurgical bonding and the capture sleeve secured to theadapter sleeve by metallurgical bonding, and said retainer being securedto the end nozzle by metallurgical bonding.

11. The fuel assembly according to claim 1, wherein a plurality of gridsare longitudinally spaced along the length of said tubular members, andthe openings therein are in alignment for receiving the fuel elementsand tubular members which bridge the space between said end nozzles; and

the upper of said grids being located adjacent said top end nozzle andthe lower of said grids located adjacent said bottom end nozzle. 12. Thefuel assembly defined in claim 11, wherein the means separatelyattaching the tubular members to the top end nozzle comprises a sleeveon each tubular member metallurgically bonded to said nozzle;

means securing each. sleeve to each tubular member;

and

each sleeve on each tubular member being metallurgically bonded to thegrid straps of the upper grid to hold the tubular members in position.

13. The fuel assembly according to claim 12, wherein the means securingthe sleeve to said member comprises integrally formed protrusions onsaid tubular member which coact with the sleeve to prevent itsdisplacement on said member.

14. The fuel assembly defined in claim 11, wherein the means separatelyattaching the tubular members to the bottom end nozzle comprises asleeve on the bottom of each tubular member;

a mechanical device connecting each sleeve to the bottom end nozzle; and

each sleeve on each tubular means being metallurgically bonded to thegrid straps of the lower grid to hold the tubular members in position.

15. The fuel assembly defined in claim 14, wherein said mechanicaldevice includes a plug coacting with the walls of the tubular member andsleeve; and

a coupler connecting the plug with the bottom end nozzle.

16. The fuel assembly according to claim 11, wherein extension meansinterconnects the outer portion of the upper grid with the top nozzle,and wherein the means attaching the tubular members to said top nozzleincludes a close flt which permits axial movement of the tubular memberstherein; v

projections on said tubular members immediately abve and below saidupper grid,

said device on each tubular member being ofa length sufficient to haveits ends bear against said projections.

17. The fuel assembly according to claim 16, wherein each deviceincludes an outwardly directed portion intermediate its length arrangedto contact adjacent tubular members through openings in the grid strapwalls. l

UNITED STATES PATENT) OFFICE CERTIFICATE OF CORRECTION Patent No.3,79l," Dated r ary 1 197 4 Inventor(s)'John F. Patterson, Elmer A.Bassler, Jr. Edward J.Ch'O y It is certified that error appears in theabove-identified patent and .that said Letters Patent are herebycorrected as shown below:

Column 6, line 62, cancel "means" and insert member Column 8, line 27,cancel "means" and insert member Signed and sealed this 10th day ofDecember 197 (SEAL) Attest:

McCOY M, GIBSON JR. C. MARSHALL DANN Attes'ting Officer I Commissionerof Patents FORM PC4050 (10-69) v ulcouwoc 00870-9" I as. covnunlr mumsomen mo-ul-au yAttest:

UNITED. STATES PATENT. OFFICE CERTIFICATE OF CORRECTION Dated bruary 12,197M Patent No. 3,791,466

Edward J .Ch'oby Inventor (s)John F. Pac terson, Elmer A. Bassler, Jr.

I It; is certified that error appears in the above-identified patent and.that said Letters Patent are hereby corrected as shown below:

"means" and insert member Column 6,. line 62, cancel and insert memberColumn 8, line 27, cancel "means Signed and seeledthis 10th day ofDecember 19%.

(SEAL) MCCOY M, GIBSON JR. C. MARSHALL DANN Attesting OfficerCommissionervof-Patents UlcOMM-DC 60870-900 Q v.1. oovnmmn "will. olnclI. o-ul-u4 FORM Po-1050 (10 69)

1. A fuel assembly for a nuclear reactor comprising an array ofgenerally parallel elongated fuel elements, a plurality of elongatedhollow tubular members containing control elements strategically locatedamong the array of fuel elements, said tubular members being composed ofa relatively low neutron capture metal, means separately attaching theopposite ends of said tubular members to top and bottom end nozzlesrespectively located at opposite ends thereof; at least one grid locatedbetween said end nozzles and formed by coacting straps which definemultiple spaced openings through which the fuel elements and tubularmembers extend; a device immovably positioned on each of said tubularmembers, said device being located between said tubular means and thewalls of said straps; and means securing said device to the inner wallsof said straps which define the opening through which each tubularmember extends, to thereby hold each tubular member in position in thefuel assembly.
 2. The fuel assembly defined in claim 1, wherein thedevice comprises a sleeve surrounding each tubular member.
 3. The fuelassembly defined in claim 2, wherein each sleeve is composed of a metalsimilar to the metal of the grid straps, and the sleeve is secured tothe grid straps by metallurgical bonding.
 4. The fuel assembly definedin claim 3, wherein the grid straps are disposed at right angles to formgenerally square openings, and each sleeve has slots therein of a sizesufficient to receive the straps during grid assembly to thereby holdthe device and tubular member immovably in the grid straps.
 5. The fuelassembly defined in claim 2, wherein spring means is secured to theinside surface of and along the length of said straps forming eachopening, said spring means being positioned to bear against each tubularmember and fuel element in the appropriate openings to prevent theirlateral displacement.
 6. The fuel assembly defined in claim 1, whereineach tubular member is deformed adjacent at least one end of said deviceto immovably fix the device on its tubular member.
 7. The fuel assemblydefined in claim 1, wherein the device is composed of a metal similar tothe metal of the tubular members and is secured to the tubular membersby metallurgical bonding.
 8. The fuel assembly defined in claim 7,wherein the grid straps have recesses therein, and the device extendsinto said recesses.
 9. The fuel assembly defined in claim 1, whereinsaid device comprises an adapter sleeve surrounding each tubular memberextending through the top nozzle, said adapter sleeve having an externalflange thereon, a retainer surrounding the adapter sleeve, said retainerhaving an internal flange engaging the external flange on the adapterslEeve, and a capture sleeve extending between the adapter sleeve andthe retainer to engage the internal flange on the retainer.
 10. The fuelassembly defined in claim 9, wherein the adapter sleeve and the capturesleeve are composed of a metal similar to the metal of the tubularmember and the retainer is composed of a metal similar to the endnozzle, and wherein said adapter sleeve is secured to the tubular memberby metallurgical bonding and the capture sleeve secured to the adaptersleeve by metallurgical bonding, and said retainer being secured to theend nozzle by metallurgical bonding.
 11. The fuel assembly according toclaim 1, wherein a plurality of grids are longitudinally spaced alongthe length of said tubular members, and the openings therein are inalignment for receiving the fuel elements and tubular members whichbridge the space between said end nozzles; and the upper of said gridsbeing located adjacent said top end nozzle and the lower of said gridslocated adjacent said bottom end nozzle.
 12. The fuel assembly definedin claim 11, wherein the means separately attaching the tubular membersto the top end nozzle comprises a sleeve on each tubular membermetallurgically bonded to said nozzle; means securing each sleeve toeach tubular member; and each sleeve on each tubular member beingmetallurgically bonded to the grid straps of the upper grid to hold thetubular members in position.
 13. The fuel assembly according to claim12, wherein the means securing the sleeve to said member comprisesintegrally formed protrusions on said tubular member which coact withthe sleeve to prevent its displacement on said member.
 14. The fuelassembly defined in claim 11, wherein the means separately attaching thetubular members to the bottom end nozzle comprises a sleeve on thebottom of each tubular member; a mechanical device connecting eachsleeve to the bottom end nozzle; and each sleeve on each tubular meansbeing metallurgically bonded to the grid straps of the lower grid tohold the tubular members in position.
 15. The fuel assembly defined inclaim 14, wherein said mechanical device includes a plug coacting withthe walls of the tubular member and sleeve; and a coupler connecting theplug with the bottom end nozzle.
 16. The fuel assembly according toclaim 11, wherein extension means interconnects the outer portion of theupper grid with the top nozzle, and wherein the means attaching thetubular members to said top nozzle includes a close fit which permitsaxial movement of the tubular members therein; projections on saidtubular members immediately abve and below said upper grid, said deviceon each tubular member being of a length sufficient to have its endsbear against said projections.
 17. The fuel assembly according to claim16, wherein each device includes an outwardly directed portionintermediate its length arranged to contact adjacent tubular membersthrough openings in the grid strap walls.